Materials Letters 164 (2016) 498–501

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Materials Letters

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Band-gap tuning of lead halide perovskite using a single stepspin-coating deposition process

Saif M.H. Qaid a, Mohammed. S. Al Sobaie a, M.A. Majeed Khan b,n, Idriss M. Bedja c,Fahhad. H. Alharbi d, Mohammad Khaja Nazeeruddin e, Abdullah S. Aldwayyan a,b

a Physics and Astronomy Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabiab King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabiac CRC, Department of Optometry, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabiad Qatar Foundation, Qatar Environment and Energy Research Institute, P. O. Box 5825, Doha, Qatare Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Science, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland

a r t i c l e i n f o

Article history:Received 13 October 2015Accepted 27 October 2015Available online 29 October 2015

Keywords:Organic–inorganic perovskiteOptical materials and propertiesLuminescenceX-ray techniquesThin films

x.doi.org/10.1016/j.matlet.2015.10.1357X/& 2015 Elsevier B.V. All rights reserved.

esponding author. Fax: þ966 14670662.ail address: majeed_phys@rediffmail.com (M.A

a b s t r a c t

In this paper, we present a study of the structural and optical properties of organic–inorganic halide-based perovskite semiconductors with band gaps varying from NIR to visible at room temperature. Thinfilms of nanocrystalline CH3NH3PbI3 with different concentrations of methylammonium iodide (MAI)have been successfully deposited onto glass substrates using single step spin-coating technique. Theprepared films have been characterized by X-ray diffraction, optical absorption spectroscopy and pho-toluminescence measurements. X-ray diffraction scans revealed that even for stoichiometric atomicconcentrations of MA:Pb:I of 1:1:3, the PbI2 phase was also present. The PbI2 phase showed a highlytextured along the (001) direction in all the prepared films, with a crystallite size in the range of ap-proximately 30–40 nm. The optical absorbance edge of CH3NH3PbI3 thin films is described in terms ofdirect transition model proposed by Tauc in the strong absorption region. The band gap of the pure PbI2film was calculated to be 2.40 eV, whereas the band gap of the pervoskite film with stoichiometric ionicratio of 1:1 PbI2 and CH3NH3I was calculated to be 1.46 eV, while absorption coefficient is �105 cm�1.Photoluminescence measurements showed a red shift in the perovskite emission with increasing the MAIconcentration, confirming the correct placement of the MA ions in the perovskite crystalline structureshowing the PbI2 octahedra. Our results confirm the existence of PbI2 phase in the highly efficientperovskite solar cells, as demonstrated in the recent publication.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

In the recent years, photovoltaics have become an active fieldfor basic research and technology. The development of new ma-terials has opened new perspectives for the production of costeffective and highly efficient photovoltaic devices. Hybrid organic–inorganic perovskite materials, such as CH3NH3PbI3 have attractedthe attention of the photovoltaic research community due to theirpromising performance as light harvesting material. The ad-vantages of these compounds have already been demonstrated asa noteworthy example of potentially useful physical propertiessuch as nonlinear optical properties, electroluminescence, organic-like mobility, magnetic properties, conductivity, and so forth [1].Recent studies report that solar cells based on CH3NH3PbI3

. Majeed Khan).

perovskite have promising photovoltaic efficiencies reaching over20% [2]. Within the past 5 years, hybrid organic–inorganic per-ovskite solar cells have been reported to achieve remarkably a highpower conversion efficiency jumping from 3.8% [2] in 2009 to20.1% [3]. The stunning high efficiencies are due to CH3NH3PbI3perovskite ideal direct band gap, large absorption coefficient, highcarrier mobility and simple fabrication techniques [4]. One of theimportant properties of the organic–inorganic perovskite for theiruse in photovolatic cells is the possibility to tune their opticalproperties (e.g., band gap).

2. Experimental details

A variety of techniques have been applied to depositCH3NH3PbI3 perovskite thin films, for instance, single-step solu-tion deposition, vacuum deposition, sequential deposition, vaporassisted solution process, and vacuum thermal co-evaporation.

Table 1Properties of CH3NH3PbI3 thin films with different (MAI) concentrations.

Sample no. Required con-centration ofCH3NH3I

Crystallizesize (nm)

Bandgap (eV)PbI2

Band gap (eV)CH3NH3PbI3

1 0.0 34.85 2.40 –

2 10 35.79 2.37 –

3 20 35.79 2.34 –

4 30 35.78 2.30 –

S.M.H. Qaid et al. / Materials Letters 164 (2016) 498–501 499

Among these methods, the sequential deposition of spin-castingfirst PbI2 followed by equimolar CH3NH3I resulted in highestpower conversion efficiency. This procedure resulted in perovskitethin films of most homogeneous morphology and the highest thin-film coverage, leading to a high performance of (PCE). In thepresent work, we successfully deposited CH3NH3PbI3 perovskitefilms onto glass substrates by spin-casting technique; and themicrostructure of films along with optical characteristics has beendiscussed.

5 40 35.77 2.22 1.706 50 34.85 2.14 1.657 60 34.84 2.04 1.628 70 33.96 1.91 1.599 80 33.23 1.85 1.55

10 90 34.91 1.81 1.5311 100 35.78 1.72 1.46

3. Results and discussion

3.1. Microstructure studies

X-ray diffraction (XRD) study was used to identify the phasesand estimation of sizes of the prepared nanocrystallites. Methy-lammonium lead iodide (CH3NH3PbI3) films were formed via spincoating of an equimolar mixture of CH3NH3I and PbI2 precursorsolutions. The appearance of strong diffraction peaks located at2θ¼14.38° and 28.59° (Fig. 1(a)) was observed corresponding tothe planes of (110) and (220), which is good in agreement with theprevious reports [5], indicating that the tetragonal perovskitestructure is formed [6]. Moreover, we also find that the impuritypeaks around 12.76°, 38.81° and 52.53° can be attributed to PbI2crystals in these samples [7]. No impurity peaks appeared in bothsteps, suggesting a complete formation of PbI2 and its controlledtransformation to CH3NH3PbI3. Based on a calculation using theScherrer equation, the average crystallite size of CH3NH3PbI3

Fig. 1. (a) X-ray diffraction patterns of CH3NH3PbI3 films with (MAI) concentrations; (bfraction; (c) α vs hν for CH3NH3PbI3 films as a function of unconverted PbI2 phase fractioconcentrations at 500 nm.

nanocrystals is tabulated in Table 1.

3.2. Optical studies

To check the variation of optical properties in the alloyed hy-brid halide perovskite, we measured the UV–visible absorptionspectra of CH3NH3PbI3 perovskites which is shown in Fig. 1(b). Theabsorption onset is red-shifted after substituting CH3NH3I, whichis associated with band gap narrowing upon methylammoniumion concentrations doping. Its absorption covers a wide range oflight from the visible to the near-IR region, indicating the forma-tion of CH3NH3PbI3 perovskite on the substrate [8]. The onset

) absorption spectra of CH3NH3PbI3 films as a function of unconverted PbI2 phasen; (d) plot of (αhν)2 vs hν for CH3NH3PbI3 thin films changes with increasing (MAI)

S.M.H. Qaid et al. / Materials Letters 164 (2016) 498–501500

absorption band of CH3NH3PbI3 perovskite can be tuned from(1.46 eV) to (2.40 eV), resulting in color tunability for colorful solarcells. The first absorption peak at 760 nm corresponds to the directband gap transition from the first valence band (VB1) to CB whilethe other at 500 nm to the transition from the second valenceband (VB2) to CB [9]. The dependence of the optical absorptioncoefficient with the photon energy has been known to help tostudy the type of transition of electrons and semiconductors'bandgap energy as well. The optical absorption coefficient (α) isevaluated from the absorption spectra using the relation [10]

α = ( )dOD

1

where α is the absorption coefficient and d is the sample thick-ness. The calculated α for the CH3NH3PbI3 perovskite film is105 cm�1 at 501 nm (Fig. 1(c)), which is approximately an order ofmagnitude higher than that previously reported [5]. They esti-mated α of CH3NH3PbI3 to be 1.5�104 cm�1 at 550 nm, while ourcalculation yields between 4.4�104 and 5.1�105 at 501 nm. Ascompared to inorganic semiconductors [11], CH3NH3PbI3 has avery similar range of absorption coefficient. Due to the reasonablyhigh α of CH3NH3PbI3 in the visible region, it is possible to form asufficiently thin yet strongly absorbing film, which is crucial to abilayer device. According to Tauc's law, the absorption coefficient isrelated to band gap (Eg) as follows [12]:

( )α( ) = * − ( )hv A hv E 2m

g

Fig. 2. (a) Plot of (αhν)2 vs hν for CH3NH3PbI3 thin films changes with increasing (MCH3NH3PbI3 thin films; (c) emission spectra of CH3NH3PbI3 with different (MAI) conCH3NH3PbI3 thin films.

where A* is a constant, which does not depend on photon energyand Eg is the optical band-gap energy. Figs. 1(d) and 2(a) show theplots of (αhν)2 vs hv for the studied thin films deposited on glasssubstrate at room temperature. The value of Eg for films is de-termined by extrapolation of the linear part of this relationship andthe interception from the abscissa (at α¼0, Eg¼hν). The values of Egfor all films deposited at room temperatures are given in Table 1.We wanted to further investigate how the optical absorption of thefilm changes with increasing (MAI) concentrations. The pure PbI2film shows a band gap of 2.40 eV, consistent with the yellow colorof PbI2. As the PbI2 film is gradually converted to the perovskite, theband gap is progressively shifted toward 1.72 eV. The deviation ofMAPbI3's band gap (1.72 eV) from that of the bulk MAPbI3 material(1.55 eV) could be explained by the effects of crystallite sizes andinterfacial interaction of MAPbI3 [13]. We can also notice the pre-sence of a second absorption in the light absorber layer, in whichthe gap gradually red shifts from 1.70 eV to 1.46 eV as the CH3NH3Iconcentration is decreased from 100% to 40%. Fig. 2(b) indicates thatdirect band gap decreases with increase of molar concentration ofthe precursor material. It could be due to the increase of density oflocalized state in the conduction band.

3.3. Photoluminescence spectroscopy

Photoluminescence (PL) measurement is a perceptive helpfultechnique to explore the optical properties of semiconductingmaterials. Fig. 2(c) shows the PL emission spectra peaks of theMAPbI3 perovskites thin films, the emission peaks of each sample

AI) concentrations at 770 nm; (b) variation of Eg with (MAI) concentrations forcentrations at room temperature; and (d) PL peak vs (MAI) concentrations for

S.M.H. Qaid et al. / Materials Letters 164 (2016) 498–501 501

exhibit a small shift to the longer wavelength side [14]. The var-iation of PL peak position vs (MAI) concentration is shown in Fig. 2(d). Note that all perovskite samples were prepared and testedunder the same conditions. It is surprising that such a slight var-iation in the composition can lead to such a remarkable differencein their PL emission intensities. The exact reason is still not clear,while we speculate that it might be associated with the orienta-tion and vibration restraint of the PbI2 in the perovskite lattice asthe MA ions substitution rate varied, confirming the correct pla-cement of the MA ions in the perovskite crystalline structureconfirming the PbI2 octahedra. Based on the above superior lightemitting properties, MAPbI3 is considered to be a promising can-didate for photovoltaic applications.

4. Conclusion

In summary, the microstructure and optical properties ofCH3NH3PbI3 perovskite thin films prepared by the single step spin-coating technique have been investigated. A remarkable red shift ofthe absorption edge was achievable by increasing the methy-lammonium iodide (MAI) concentrations (0rxr100). The reportedCH3NH3PbI3 perovskite is promising for low-cost thin-film solar cellsdue to its tunable bandgap between 1.46 and 2.40 eV and a largeabsorption coefficient of over 105 cm�1. The slightly red-shifted andhomogenously broadened PL spectra exhibit features consistent withphonon coupling as the dominant spectral broadening mechanism ofthe emission from CH3NH3PbI3. Our findings underline the potentialof organometal halide perovskites for a wider range of optoelectronicapplications beyond photovoltaic energy conversion.

Acknowledgement

This Project was funded by the National Plan for Science,Technology and Innovation (MAARIFAH), King Abdulaziz City forScience and Technology, Kingdom of Saudi Arabia, Award Number(ENE1474-02) and the authors extend their appreciation to theDeanship of Scientific Research at King Saud University for fundingthis work through research group no RGP-265.

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